1. Field of the Invention
The present invention relates to the field of current source arrays for use in thermometer coded current steering digital to analog converters and the like.
2. Prior Art
Arrays of equal value current sources are used to define the most significant bits in high-speed digital to analog converters (DACs). Digital to analog conversion is achieved by steering these currents to one of two analog outputs, typically terminated in resistors. Accurate matching between these current sources, typically implemented with MOS transistors, is essential to achieve a DAC that is highly linear without being trimmed or calibrated.
The individual current sources of a DAC are mismatched for a number of reasons. Random fluctuations in threshold voltage and charge carrier mobility causes a random mismatch between the current sources. Gradients in oxide thickness, mechanical stress and resistive voltage drops in supply lines cause linear, parabolic and higher order gradients across the current source array, resulting in a nonlinear DAC transfer function.
Therefore, the layout of thermometer coded most significant bits (MSBs) for a current steering DAC must be done with utmost care to avoid systematic errors due to linear and higher order process gradients. It is also important that the layout be practical and correct in other respects as outlined below:
1. The amount of routing must be the same for all current sources, such that the output capacitance is the same for all current sources.
2. The amount of routing required to implement the layout must not be excessive, such that the (overhead) output capacitance caused by routing is low.
3. The layout must compensate for linear and parabolic process gradients.
4. A possibility to integrate the LSB current sources in the layout for the MSB sources is valuable.
It is useful to be able to access the outputs of the array from at least two sides to be able to implement trim or calibration current sources on one side and DAC switches on the other side, as illustrated in FIG. 1.
The number of current sources to be laid out is nominally 2Nxe2x88x921, where N is the number of thermometer coded most significant bits (MSBs) in the DAC. In order to compensate for process gradients, each source is normally split into m smaller unit current sources, and these are distributed into an array of 2Nxc3x97m unit sources in such a manner that process gradients are compensated. One way of doing this is by distributing the current sources evenly in both the vertical and horizontal directions, typically with 2N rows and 2N columns (m=2N). An obvious example of such a layout is described in U.S. Pat. No. 5,568,145. An example layout is shown in FIG. 2. The current sources are numbered from 1 to 16. In this case, each current source is split into 16 unit current sources, giving a total current source array size of 256 sources.
The output routing is illustrated in FIG. 3 for an example array of 8 current sources. The layout has mostly local interconnect, which keeps the output capacitance low. To match the output capacitances of all sources accurately, it is necessary to add one set of interconnect to the right or left side of the array. The patent also describes integrating the LSBs of the DAC in a column or a row located in the center of the array, as shown in FIG. 3.
The technique of the foregoing patent cancels first and second order gradients in horizontal and vertical directions, but does not cancel second order gradients due to parabolic gradients in other directions (rotated parabolic gradients). The rotated parabolic gradients in such a layout cause a parabolic error across the current source array. This will typically cause an xe2x80x9cS-shapedxe2x80x9d integral nonlinearity error, and hence a third order harmonic distortion in the output of the DAC.
Current source arrays having a plurality of current sources arranged in an array of columns and rows are disclosed. The outputs of the current sources in even rows of the first column of an array are connected to the output of a current source in each of the other columns located along a first diagonal through the array from the respective current source in the first column. Also the outputs of the current sources in odd rows of the first column of the array are each connected to the output of a current source in each of the other columns located along a second diagonal through the array from the respective current source in the first column, the second diagonals being in an opposite diagonal direction from the first diagonals. When used in a current steering thermometer DAC, preferably but not necessarily, the current sources for the least significant bits are located on a main diagonal of the array.